Nanostructured layers, methods of making nanostructured layers, and application thereof

ABSTRACT

One embodiment of the invention provides a nanostructure layer, comprising: a first population of semiconductor nanocrystals forming electron transport conduits; a second population of semiconductor nanocrystals forming hole transport conduits; and a third population of semiconductor nanocrystals capable of at least one of the following: absorbing light or emitting light.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 12/001,581, filed Dec. 11, 2007 (currently allowed)now U.S. Pat. No. 7,785,657, which claims the benefit of U.S.Provisional Application Ser. No. 60/874,043 filed on Dec. 11, 2006. Thedisclosures of the Application and the provisional application areincorporated herein by reference in their entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to a composition of and methodof making nanostructured layers derived from semiconductor nanocrystalsthat can be incorporated into solar cells, photodetectors, lasers, lightemitting diodes, and other optoelectronic devices.

BACKGROUND

Renewable energy from the sun has great potential in reducing dependencyon fossil fuels while also providing a cleaner,non-green-house-gas-producing method of power generation. A basiclimitation of solar power, however, is its high cost relative to otherenergy sources. Decreasing the cost per watt can be made possible byimproving efficiencies and by decreasing the manufacturing costs ofsolar cells. Efficiency gains can be realized by increasing thepercentage of the solar spectra that is captured and by decreasing lossmechanisms due to charge carrier thermalization, electron/holerecombination, and resistive contact losses. Decreasing solar cellmanufacturing costs can also be made possible by utilizing less costlysolar cell substrate materials and increasing manufacturing throughputthrough area device fabrication.

Although the theoretical solar conversion efficiency is 66%, traditionalsingle junction solar cells have a maximum efficiency of only 33%, andin practice, rarely achieve efficiencies greater than 18%. Tandem cells,comprising a multiple stack of junctions where each junction isoptimized for progressively longer wavelengths, have achievedsignificantly higher efficiencies but at even higher costs.Semiconductor quantum dot (QD) based solar cells are ideally suited toincrease conversion efficiencies because they have size andcompositionally tunable bandgaps and broadband absorption. They are alsoideally suited to decrease fabrication costs because they can bedeposited over large area planar and nonplanar substrates using low costspin coating or roll-to-roll processes. “Multi Junction” solar cellscomprising layers of quantum dots each with a varying size and/orcomposition can potentially achieve the greater efficiencies of a tandemcell but at far lower processing costs. Multiple exciton generation(MEG) in lead sulfide quantum dots supplied by Evident Technologies cancreate more than one electron hole pair per absorbed photon providedthat the photon energy is more than twice that of the quantum dotbandgap. By harnessing the MEG process the efficiency loss throughcharge carrier thermalization could be mitigated and high efficiencysolar conversion realized.

Research on quantum dot based solar cells has been ongoing for sometime, but it has yet to result in high efficiencies. The vast majorityof the research efforts have focused on implementing the nanocrystalcolloids into polymer (MEH-PPV, polythiophene, PFO, etc.) solar cellswhere the quantum dots are either dispersed within a semiconductorpolymer, between semiconductor polymer layers, or between asemiconductor polymer layer and an electrode. The interband statespresent at the inorganic QD/organic polymer interface and QD/polymerband offset result in significant charge carrier recombination, whichcauses loss of efficiency. The efficiency loss is exacerbated by theorganic surfactant layer that envelops colloidal quantum dots.Surfactants enable the particles to disperse in solution, co-solvatewith the conjugated polymer, and deposit as a film from solution.Typical surfactants can include TOPO, alkane thiols, and aliphaticamines, all of which are insulators. Any charge transfer from the QDs tothe surrounding polymer within solar devices is accomplished through ahighly inefficient tunneling process that limits overall deviceefficiency.

Semiconductor nanocrystals, otherwise known as quantum dots, are tinycrystals typically made of II-VI, III-V, IV-VI, and I-III-VI materialsthat have a diameter between 1 nanometer (nm) and 20 nm. In the strongconfinement limit, the physical diameter of the nanocrystal is smallerthan the bulk excitation Bohr radius causing quantum confinement effectsto predominate. In this regime, the nanocrystal is a 0-dimensionalsystem that has both quantized density and energy of electronic stateswhere the actual energy and energy differences between electronic statesare a function of both the nanocrystal composition and physical size.Larger nanocrystals have more closely spaced energy states and smallernanocrystals have the reverse. Because interaction of light and matteris determined by the density and energy of electronic states, many ofthe optical and electric properties of nanocrystals can be tuned oraltered simply by changing the nanocrystal geometry (i.e. physicalsize).

Precise control over nanocrystal size, shape, composition, and surfacechemistry allows for the rational engineering of amorphous (i.e. randomdistribution of colloidal particles within the solid) or crystalline(spatially ordered array of nanocrystals) nanocrystal based colloidalsolids. Nanocrystals can be defined by their composition, size, shape,and surface chemistry. All nanocrystals, including semiconductor quantumdots are inherently insoluble without the presence of organic cappingmolecules referred to as ligands. Ligands have two functional chemicalgroups, one of which coordinates to the metal atoms comprising thesurface of the quantum dot and the other which allows for thenanoparticles to disperse within a given solvent. The strength at whichthe ligands bind to the nanocrystal surface is dependent on the chemicalpotential between metal atom and the specific metal coordinating groupwhile the compatibility with a given solvent is dependent upon themagnitude of the polarity or ionization of the opposing moieties. Commonmetal coordination groups can include phosphine, phosphine oxide, amine,carboxyl, and thiol groups.

Efforts to improve the quality and complexity of assembled colloidalnanocrystals continue to this day. The ability to produce a vast numberof nanostructured thin film metamaterials derived from binarypopulations of colloidal nanocrystals where the particles were selfassembled into ordered crystalline lattices has been demonstrated. Ithas been shown that many types and compositions of nanoparticles can beused including semiconductor quantum dots, several forms of metalnanoparticles, and oxides. By altering the ratio of diameters and therelative concentrations of nanoparticles comprising the nanostructuredfilm many different crystal structures can be formed. Nanostructuredlayers comprised of single or binary populations of nanoparticles whereall the constituents have the same or nearly the same diameter can packneatly into a hexagonal close packed structure, whereas if the ratio ofdiameters and/or the prevalence of the various constituents is changedfrom 1:1 continuously to 1:13 cubic, orthorhombic, and tetragonalsymmetries are produced. It has also been found that dipole-dipole andvan der Waals forces play a significant role in the symmetries produced.

The limitations of present quantum dot/polymer solar cells, LEDs, andother optoelectronic devices result from energy transport inefficienciesat the organic/inorganic interface. These detrimental effects can belargely mitigated by employing all quantum dot based nanostructured thinlayers that are devoid of organic materials in the active region of theoptoelectronic devices.

SUMMARY

Aspects of the present invention include inorganic nanostructured layersderived from three or more colloidal quantum dot (semiconductornanocrystal) populations that can be assembled and processed on asubstrate such that each nanocrystal can be in electrical contact withadjacent nanocrystals in the nanostructured film. Nanostructured layerscan be constructed such that the first population of nanocrystalscreates electron transport conduits, the second population ofnanocrystals forms hole transport conduits, and the other populations ofnanocrystals can be used for light absorption or light emissiondepending on the type of optoelectronic device in which the film isincorporated. When incorporated into optoelectronic devices, thenanostructured layers can perform charge transport as well as chargerecombination in light emitting devices, or exciton creation and chargeseparation in light absorbing devices such as solar cells andphotodetectors. In conventional semiconductor devices, these functionsare performed by distinct semiconductor layers that are in contact toform a junction.

Uses of the nanostructured layer can include solar cells, photodetectorsand photodetector arrays, and devices where the nanostructured layersserve the light absorption, charge separation, and charge transportfunctions. The nanostructured layers may also be used in LEDs, lasers,and other similar optoelectronic devices where the layer can serve thecharge transport, charge recombination, and light emission functions.The nanostructured layer may be fabricated over a large area usinginexpensive liquid phase deposition techniques, self assembly, andthermal sintering or chemical cementing techniques described herein.Optoelectronic devices comprising the nanostructured layers can exhibitincreased charge carrier mobilities and reduced nonradiativerecombination losses in comparison to QD/polymer hybrid devices due tothe elimination of the inorganic/organic interface at the semiconductornanocrystal/polymer or ligand boundary.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a nanostructured layer embodying aspects ofthe present invention.

FIG. 2 a is the band diagram of a photovoltaic device showing therelative bandgaps and bandoffsets of the nanocrystals comprising thenanostructured layer.

FIG. 2 b is the band diagram of a light emitting device showing therelative bandgaps and bandoffsets of the nanocrystals comprising thenanostructured layer.

FIG. 3 a is an illustration of a nanostructure layer embodying aspectsof the present invention. The nanostructure layer is configured toabsorb light.

FIG. 3 b is an illustration of a nanostructure layer embodying aspectsof the present invention. The nanostructure layer is configured to emitlight.

FIG. 4 is a chart of the bandgaps and band offsets of a selection ofsemiconductor material.

DETAILED DESCRIPTION

Composition of the Layer

FIG. 1 illustrates an all inorganic nanostructured layer 100 embodyingaspects of the present invention. The nanostructured layer 100 can becomprised of three or more populations of semiconductor nanocrystals(quantum dots) 110 a-c. The nanocrystals 110 a-c can be in electricalcontact with adjacent nanocrystals, and the nanostructured layer 100 canbe devoid of residual organic moieties. The nanostructured layer 100 canbe constructed such that the first population of nanocrystals 110 acreates electron transport conduits, the second population ofnanocrystals 110 b forms hole transport conduits, and the otherpopulations of nanocrystals 110 c can be used for light absorption orlight emission depending on the type of optoelectronic device in whichthe film is incorporated.

Thus, the proposed nanostructured film illustrated in FIG. 1 can bedescribed as an interpenetrated type II heterojunction sensitized withsemiconductor nanocrystals that retain size and compositionallydependent energy states (i.e. quantum confinement). The alignment of theband gaps (and modified bandgaps due to quantum confinement effects) canbe selected in order to create, separate, and transport charge carriersvia a band structure shown in FIG. 2 a for photovoltaic devices and FIG.2 b for light emitting devices employing nanostructured layers, wherethe relative height and separation between elements 201 a and 201 b and202 a and 202 b illustrate the bandgaps and bandoffsets of the first andsecond populations of nanocrystals.

FIGS. 3 a and 3 b illustrate further aspects of embodiments of thepresent invention. FIG. 3 a illustrates a nanostructure layer configuredto absorb light 140. An aspect of the present invention includes havingenough nanocrystals of the first population 110 a in electrical contactwith each other to form an electron transport conduit to a firstelectrode, such as an anode 120, and having enough nanocrystals of thesecond population 110 b in electrical contact with each other to formhole transport conduits to a second electrode, such as a cathode 130,when light 140 is absorbed by the third population 110 c ofnanocrystals. The electrons 111 and holes 112 of FIGS. 3 a and 3 b areshown as “e” and “h” respectively. Because the other populations ofnanocrystals 110 c comprising the layer provide exciton formation inlight absorbing optoelectronic devices, it is not necessary that theyare in electrical contact with nanocrystals of the same population.However, nanocrystals of the third (or other) populations 110 c must bein electrical contact with nanocrystals of both the first 110 a andsecond 110 b populations of nanocrystals. In order to achieve thiscondition in a three dimensional layer the ratio of the first 110 a andsecond 110 b populations of nanocrystals can be approximately 1:1 andcomprise at least approximately 40% and up to approximately 70% of thelayer while the ratio of each of the other populations of nanocrystals(third 110 c, optional fourth, optional fifth, optional sixthpopulations etc.) to the sum of the first 110 a and second 110 bpopulations can be between 60% and 30%. Direct electrical contact mayrefer to direct physical contact made between the nanocrystal cores ofadjacent nanocrystals as a result of sintering or chemical cementing, ormay refer to a condition where the nanocrystal cores are in close enoughproximity (approximately <50 angstroms) where Forster or Dexter energytransfer between adjacent nanocrystal cores may occur. Creatingelectrical contact between adjacent nanocrystals of the same populationcan result in a loss of the quantum confinement characteristicsindicative of nanoscale semiconductor quantum dots. Thus the first 110 aand second 110 b populations of semiconductor nanocrystals might notexhibit energy levels associated with 3-d electron/hole energyquantization (although they may exhibit confinement in 2 dimensions).However, the third (and other) populations 110 c of nanocrystals thatare not in electrical contact with adjacent nanocrystals of the samepopulations can retain the discrete energy levels associated withquantum confinement of charge carriers in all 3 dimensions.

In one embodiment of the present invention, the first 110 a and second110 b populations of nanocrystals can exhibit a band offset from oneanother such that the conduction and valence bands of the firstpopulation (electron conducting) is lower than the conduction andvalence bands of the second population (hole conducting).

FIG. 3 b illustrates a nanostructure layer similar to the one in FIG. 3a, but instead of being configured for a light absorbing device (as inFIG. 3 a), it configured for light emitting devices (LEDs, lasers etc.).For light emitting devices, the other populations of nanocrystals 110 ccan have a conduction band that is lower than that of the firstpopulation 110 a of nanocrystals and valence bands higher than that ofthe second population 110 b of nanocrystals. In light absorbing devices(FIG. 3 a) such as solar cells, photodetectors, and photodetectorarrays, the other populations 110 c of nanocrystals can have aconduction band that is higher than the first population 110 a (but nothigher than the conduction band of the second population 110 b) ofnanocrystals, and valence bands that are lower than the valence bands ofthe second population 110 b of nanocrystals (but not lower than thevalence bands of the first population of nanocrystals).

Optoelectronic devices comprising the nanostructured layers can exhibitincreased charge carrier mobilities and reduced nonradiativerecombination losses in comparison to QD/polymer hybrid devices due tothe elimination of the inorganic/organic interface at the semiconductornanocrystal/polymer or ligand boundary. By sintering or chemicallycementing the assembled ordered quantum dots arrays, vast improvementsin charge mobilities can be achieved.

A limited set of bulk bandgaps and band offsets of various semiconductormaterials of which quantum dots are composed is shown in FIG. 4. Becausequantum confinement can result in a tunable blue shifted bandgap, thereis significant flexibility in the material compositions that can bechosen. Example materials having bandgaps that may be used to build thedesired sensitized tune II heterostructure described earlier can bechosen from FIG. 4 or the list of nanocrystal compositions given below.A nonlimiting example of a three component nanocrystal system that couldbe used in the interpenetrated network of sensitized type IIheterostructures includes InP as a light harvesting multiple excitongenerating nanocrystal, CdSe as the electron conducting quantum dot, andCdTe as the hole conducting nanocrystal. All three nanocrystalcomponents may be selected from II-VI, III-V, IV-VI, and I-III-VIsemiconductors and their alloys including the following compositions:PbS, PbSe, PbTe, GaP, InP, InGaP, InSb, AlInN, CuInGaS, CuInGaSe,ZnCuInGaS, CdSe, CdS, CdTe, ZnS, ZnSe, HgTe, GaN, InGaN etc. where thecore nanocrystal may or may not be coated with one or more layers ofsemiconductor shell. Nanocrystals comprising TiO₂, ZnO and otherwideband gap semiconductors may also be used as one or more of thepopulations comprising the nanostructured layer.

Method of Manufacture

A method of fabricating the quantum dot based all-inorganicnanostructure film discussed above may contain the following steps:

1. Synthesizing the one or more species of colloidal quantum dots(semiconductor nanoparticles) through a process such as a liquid phasechemical process.

2. Modifying the surfaces of the quantum dots with volatile organicmolecules. A nonlimiting example of a volatile organic ligand caninclude pyridine.

3. Combining the species of quantum dots in a solvent at appropriateratios.

4. Assembling ordered quantum dot solids on the lower electrode layer byslow precipitation of QDs through controlled evaporation of the solventfrom the QD dispersion.

5. Removing the volatile organic molecules on the quantum dot surfacesthrough a thermal heating process.

6. Sintering the quantum dots assembled on the substrate together toform a contiguous low defect nanostructured film capable of absorbingthe appropriate wavelengths of light, separating the charge carriers,and effectively transporting the charge carriers to opposite electrodes.Sintering can be facilitated by exposing the nanostructured layer toelevated temperatures that cause the surfaces of adjacent nanocrystalsto fuse. This temperature can be significantly lower for nanoparticlesthan for bulk materials of the same composition. Rapid thermal annealingmay be employed to avert the potential of elemental diffusion during thesintering process.

7. Post processing the sintered nanostructured film with compounds thatcan intercalate into any voids within the film and passivate residualdefects. Nonlimiting examples include hydrazine, bidentate amines, suchas ethylene diamine and pyrazine.

8. Application of the upper electrode via thin film deposition methodssuch as sputtering and evaporation known in the art.

Alternatively, improved nanoparticle interfaces within thenanostructured thin film may be achieved via a chemical cementingapproach rather than through sintering. In this approach thenanoparticle cores can be synthesized with or later combined with excessprecursors. The nanoparticle dispersion(s) with excess precursors can bedeposited and assembled on a substrate and further heated to drive offvolatile solvents and ligands while simultaneously reinitiating thechemical synthetic pathways that deposit semiconductor materials withinthe interstices between the assembled nanoparticles. Through such aprocess the nanoparticles comprising the nanostructured thin layers maybe “glued” together with semiconductor material.

Another example of a chemical cementing approach can be useful in theenhancement of semiconductor grain size. As the nanoparticles are heatedto the annealing temperature, they can fuse together to form a largegrain. As the grain size grows, the migration of the grain boundary canbecome sluggish. Ultimately, the grain stops growing. An aspect of thepresent invention includes allowing the grain size to continue to growupon the treatment of chemical cementing technique. To amplify, thenanoparticle dispersion(s) are deposited and assembled on a substrateand further annealed up to a temperature. Due to the presence ofligands, particle size and grain boundaries, the resultant film islikely porous. Then, the film is infiltrated with another nanoparticledispersion(s), followed by another annealing step. Such approachprovides further nutrients for the grain to grow and the grains to fuseat annealing temperature, and so alleviates the grain boundary problem.

An extension of the chemical cementing described above is to infiltratea different nanoparticle dispersion into the annealed film. Suchapproach can provide a method to create a nanoscale n-p junction or“heterojunction” solar cell.

Typical precursor compounds used in semiconductor nanoparticle synthesiscomprise the elements destined to be incorporated within thenanoparticle where the elements typically incorporated intosemiconductor nanoparticles come from the metal, chalcogenide, orpnictide families. The portion(s) of the precursor compounds notincorporated into the growing nanocrystals can be denoted as the carrierportion. Nonlimiting examples of the carrier portions of precursors caninclude anions, (if metal salts are used as a subset of precursors),ligands, organic complexes etc. In this embodiment, the carrier portionof the excess precursors as well as the ligands that envelop the growingnanocrystal can have low vapor pressures (i.e. can be volatile) or canbe easily eliminated by a further solvent washing step. The carrierportions of the excess precursors or the ligands that envelop thegrowing nanoparticle can comprise or have associated with them ions thatfacilitate crystal growth. For example sodium (Na) ions have been shownto facilitate crystal growth in CIS and CIGS based solar cells and thechlorine ions (Cl) derived from cadmium chloride have been shown tofacilitate crystal growth in thin film CdTe solar cells. Nonlimitingexamples of suitable precursors can include pyridine, pyrazine,picolinic acid, short chain amine coordinated Cd, Pb, Zn, In, Cu, Ga,Hg, Te, Ag, Au, etc.; or chloride acetates of Cd, Pb, Zn, In, Cu, Ga,Hg, Te, Ag, Au.

In this embodiment of the present invention, the method of thefabrication of the quantum dot based all-inorganic nanostructure filmdiscussed may contain the following steps:

1. Synthesizing one or more species of colloidal semiconductornanocrystals utilizing volatile ligands and an excess of precursors.

2. Combining the species forms of quantum dots in a solvent atappropriate ratios.

4. Assembling ordered quantum dot solids on the lower electrode layer byslow precipitation of QDs through controlled evaporation of the solventfrom the QD dispersion.

5. Removing the volatile organic molecules on the quantum dot surfacesand reacting the excess precursors within the interstitial spacesbetween assembled nanoparticles in order to deposit semiconductormaterial between them through a thermal heating process.

6. Optional further low temperature sintering of the quantum dotsassembled on the substrate together to form a contiguous low defectnanostructured film capable of absorbing the appropriate wavelengths oflight, separating the charge carriers, and effectively transporting thecharge carriers to opposite electrodes.

7. Optional post processing of the sintered nanostructured film withcompounds that can intercalate into any voids within the film andpassivate residual defects. Nonlimiting examples include hydrazine,bidentate amines, ethylene diamine.

8. Application of the upper electrode via thin film deposition methodssuch as sputtering and evaporation known in the art.

Applications

An all inorganic nanostructured layer, such as the one shown in FIG. 4a, may be incorporated into photovoltaic devices including solar cells,photodetectors, and photodetector arrays where the layer is depositedbetween a cathode 130 and anode 120. In these applications, the thirdpopulation 110 c of nanocrystals which retain their respective quantumconfinement characteristics provide the light absorption function viaphotogenerated exciton formation. The electrons 111 and holes 112comprising the excitons are separated into the electron and holetransporting conduits formed by the sintered or cemented first 110 a andsecond 110 b nanocrystal populations, whereupon they are transported tothe anode 120 and cathode 130 respectively. In one embodiment of theinvention the third population 110 c of nanocrystals exhibit multipleexciton generation where incident photons (from the light source 140)having energy greater than twice the lowest energy level (1s-1s) of thenanocrystals create more than one exciton when they are absorbed. Inanother embodiment of the invention, more than three populations ofnanocrystals can be incorporated into the nanostructured layer, wherethe third, fourth, fifth etc. populations each have a different bandgapand thus have different absorption spectra. In both embodiments, thefinal structure can be best described as an interpenetrated network ofsensitized type II heterostructures where multiple exciton generationoccurs within a narrow bandgap quantum dot light harvester and where theelectrons and holes are separated to the hole conducting and electronconducting components.

The combined effects of multiple exciton generation as well as light 140absorption in the electron and hole conducting QDs can greatly increasethe light 140 capturing ability of the resultant solar cell and greatlydecrease efficiency losses due to thermalization of the photoexcitedcharge carriers. Thus, improvements of photovoltaic conversion can beachieved by increasing charge pair generation coupled with improvedcharge mobility through sintering/chemical cementing and post sinteringpassivation steps.

The present invention combines the best features of inorganicphotovoltaic devices and organic PV devices. Inorganic solar cells haveexcellent charge transport and light absorption properties while organicsolar cells are capable of low cost, large area liquid phase processing.Improved solar conversion efficiencies over that of the quantumdot/polymer solar cells demonstrated in the literature are possible byincreasing charge mobility through the quantum dot layer by eliminatingthe inorganic/organic interface altogether.

In another embodiment of the present invention, shown in FIG. 4 b, thenanostructured layer is used in light emitting devices, such as lightemitting diodes, light emitting diode arrays, laser diodes etc. In lightemitting applications electrons 111 and holes 112 are transported fromthe cathode 130 and anode 120 via electron and hole conduits formed fromnanocrystals of the first 110 a and second 110 b populations. Electrons111 and holes 112 recombine within the third population 110 c ofsemiconductor nanocrystals resulting in light 140 emission, where thewavelength of the emitted light 140 is dependent upon the size andcomposition of the third population 110 c of nanocrystals. The peakwavelength may be in the UV, visible or infrared portions of thespectrum. In another embodiment of the invention more than threepopulations of nanocrystals comprise the nanostructured layer, where thethird, fourth, fifth and other populations (other than the first 110 aand second 110 b populations) each have different size andcompositionally dependent bandgaps such that each population ofnanocrystals emits light 140 with a different peak wavelength. Thus, thecombination of the light emitted 140 from all light emitting populationsof nanocrystals may create white light, broadband or polychromaticinfrared light, or polychromatic visible light (i.e. pinks, purples,pastels and other colors).

For all optoelectronic devices, optional electron barrier layers orother layers that inhibit the transport of charge carriers to theinappropriate electrode or facilitate their transport to the appropriateelectrode may be interposed between the nanostructured layer andelectrodes. Nanostructured layers may be incorporated as one of asuccession of vertically deposited layers on a substrate forming a“sandwich” or may be deposed between interdigitated electrodes where theanode and cathode lie laterally to the nanostructured layer.

The previous description of embodiments is provided to enable a personskilled in the art to make and use the present invention and are notintended as being limiting. Each of the disclosed aspects andembodiments of the present invention may be considered individually orin combination with other aspects, embodiments, and variations of theinvention. In addition, unless otherwise specified, none of the steps ofthe methods of the present invention are confined to any particularorder of performance, and it is contemplated that steps may be added orremoved. Various modifications to these embodiments will be readilyapparent to those skilled in the art. Therefore, the present inventionis not intended to be limited to the embodiments described herein but isto be accorded the widest scope defined only by the claims below andequivalents thereof.

1. A nanostructure layer, comprising: a first population ofsemiconductor nanocrystals forming electron transport conduits, thefirst population of semiconductor nanocrystals having first valence andconduction bands; a second population of semiconductor nanocrystalsforming hole transport conduits, the second population of semiconductornanocrystals having second valence and conduction bands; and a thirdpopulation of semiconductor nanocrystals, the third population ofsemiconductor nanocrystals having third valence and conduction bands;wherein the first, second, and third valence and conduction bands areeach different and wherein the first, second, and third populations forman interpenetrated network.
 2. The nanostructure layer of claim 1,wherein the third population of semiconductor nanocrystals provideeither exciton formation or recombination.
 3. The nanostructure layer ofclaim 1, wherein the ratio of the first population and second populationis approximately 1:1.
 4. The nanostructure layer of claim 1, wherein thefirst population and second population comprise between 40% and 70% ofthe layer.
 5. The nanostructure layer of claim 1, wherein nanocrystalsof the third population have a conduction band that is higher than aconduction band of nanocrystals of the first population and lower than aconduction band of nanocrystals of the second population, thenanocrystals of the third population further having a valence band thatis lower than a valence band of nanocrystals of the second populationand higher than a valence band of nanocrystals of the first population.6. The nanostructure layer of claim 1, wherein the first population andthe second population have offset bandgaps, and the third population hasa bandgap that is smaller than the bandgap of the first population andsmaller than the bandgap of the second population.
 7. The nanostructurelayer of claim 1, wherein the seimconductor nanocrystals of the thirdpopulation are capable of absorbing electromagnetic radiation andcreating pairs of electrons and holes, the electrons being transportedto semiconductor nanocrystals of the first population and the holesbeing transported to semiconductor nanocrystals of the secondpopulation.
 8. The nanostructure layer of claim 1, wherein semiconductornanocrystals of the third population are capable of absorbing lightthrough a multiple exciton generation process.
 9. The nanostructurelayer of claim 1, wherein semiconductor nanocrystals of the thirdpopulation are capable of recombining electrons and holes transportedfrom the first population and the second population of semiconductornanocrystals to emit light of a specific wavelength.
 10. Thenanostructure of claim 9, wherein the semiconductor nanocrystals of thethird population have a conduction band lower than a conduction band ofnanocrystals of the first population, the nanocrystals of the thirdpopulation further having a valence band higher than a valence band ofnanocrystals of the second population.
 11. The nanostructure of claim 1,wherein the semiconductor nanocrystals of the first population compriseCdSe, the semiconductor nanocrystals of the second population compriseCdTe, and the semiconductor nanocrystals of the third populationcomprise InP.
 12. The nanostructure of claim 1, wherein thesemiconductor nanocrystals of the first, the second, and the thirdpopulations are selected from the group consisting of II-VI, III-V,IV-VI, and I-III-VI semiconductors.
 13. The nanostructure layer of claim1, wherein the third population of semiconductor nanocrystals is capableof emitting light.